Photopolymerization of unsaturated compositions wherein a photoinitiating compound is included in the polymerizable mass is well known in the art. The process has many advantages over thermal polymerization and is particularly useful where long holding life combined with rapid hardening at low temperature is desirable. Photoinitiating compounds must absorb light and utilize the energy so acquired to initiate polymerization.
A large number of compounds have been found useful as photoinitiators for the polymerization of unsaturated compounds. Among those heretofore in most common usage in industry are the benzoin ethers of primary and secondary alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol and isobutyl alcohol. Also, compounds such as phenyl glyoxal and 1-phenyl butane-1,2-dione are disclosed as photosensitizers in U.S. Pat. No. 2,413,973. Additionally, various acetophenone compounds such as 2,2-diethoxyacetophenone are claimed to have photoinitiating capability in U.S. Pat. No. 3,715,293.
While particular industrial applications often dictate certain requisite characteristics, the primary determinants of universal application in the selection of a suitable photoinitiating compound are its level of reactivity and its effect upon storage stability when combined with the photopolymerizable medium wherein it is to function. This latter characteristic is significant in view of the desirability of one-component systems which will not gel prior to use.
While compounds in common use as photoinitiators do effect rates of polymerization which are industrially acceptable and render photopolymerization superior to thermal polymerization in various applications, methods of achieving increased polymerization rates with increased stability are constantly being sought. Improved photoinitiators are particularly desirable since photopolymerization techniques are gaining increasingly widespread acceptance due to the inherently lower equipment costs, reduction of volatile emissions and reduced energy consumption which attend their use.
Collaterally, the ethers of benzoin, which are widely used as photoinitiation compounds, are not wholly satisfactory with regard to the one-component system storage stability factor. Any unsaturated system to which a benzoin ether is added has considerably diminished dark storage stability and will gel prematurely. Various attempts have been made to remedy this deficiency of the benzoin compounds by including stabilizing additives in the polymerization system. Thus, U.S. Pat. No. 3,819,495 discloses the addition of organic chlorine containing compounds and copper compounds as a stabilization system while U.S. Pat. No. 3,819,496 teaches the use of organic chlorine compounds with iron and/or manganese compounds for that purpose. Many other stabilizers have been suggested and, while some improvements have been achieved in the stability of unsaturated systems containing benzoin-type photoinitiators, the necessity of incorporating stabilization compositions raises the cost of such systems appreciably while the results are still not wholly satisfactory.
Another highly desirable characteristic of a photoinitiating composition is its capacity to function acceptably in polymerizable systems which contain various pigments. This attribute is significant commercially because inorganic pigments are one of the prime components of surface coating systems and contribute directly to the usefulness of such systems by virtue of their protective function, their decorative or artistic function and other miscellaneous functions. The white opaque pigments, characterized by titanium dioxide, are the most important single group of pigments in use because of the predominance of white as a color and because of the need to use white pigments in producing many tints and light hues of color.
With regard to rate of polymerization, the resultant surface texture of the polymerized, pigmented coating and the effect of the photoinitiating compound on the color itself, none of the most widely used photoinitiating compounds is wholly acceptable in titanium dioxide pigmented unsaturated systems.
It is an object of the present invention to introduce the use of a novel class of polymerization photoinitiation compounds of enhanced reactivity.
Another object of the invention is to provide photoinitiatingcompounds which can be combined with unsaturated compounds to form one-component polymerizable systems not subject to premature gelation.
A further object of the invention is the provision of pigmented photopolymerizable compositions wherein a photoinitiator of the invention reacts acceptably while not adversely affecting the surface characteristics or the color of the resulting polymerized product.